A planetary gear comprises a planet wheel carrier, a sun shaft, a gear ring, and planet wheels supported by the planet wheel carrier so that the planet wheels are meshing with the sun shaft and with the gear ring. Bearings of the planet wheels can be splash lubricated so that the bearings are surrounded by droplets of oil when the planetary gear is operating. It is also possible that the bearings are lubricated with pressure-feed lubrication so that the planet wheel carrier comprises oil channels for conducting oil to the bearings. The pressure-feed lubrication is advantageous because an oil circulation path that comprises the above-mentioned oil channels can be provided with an oil filter so as to keep the oil clean. Furthermore, the oil circulation path can be provided with temperature control means for controlling the temperature of the oil to be in a desired temperature range. Furthermore, the above-mentioned oil channels can be designed so that the oil is directed to most critical places in the bearings and other parts of the planetary gear.
An inherent challenge related to pressure-feed lubrication of the kind mentioned above is the need to supply oil from a stationary part of a planetary gear to a rotating planet wheel carrier. On one hand, oil leakage which takes place in an oil supply connection between the stationary part and the rotating planet wheel carrier should be so small that sufficient oil pressure can be maintained in oil channels of the planet wheel carrier. On the other hand, an oil supply connection where the leakage is sufficiently small has its own challenges. In cases where the oil supply connection is contactless, clearances of the oil supply connection between the stationary part and the rotating planet wheel carrier have to be small and thus there is a risk of unintentional mechanical contacts which may damage the surfaces touching each other. In cases where the oil supply connection comprises a seal element having mechanical contacts with both the stationary part and the rotating planet wheel carrier, the seal element may increase the need for maintenance work and thereby the operating costs of the planetary gear.